Historically, integrated circuit (IC) display has been realised by means of an IC, an optical driver which may be integral with the IC or a discrete component, and a discrete optical emitter. With recent advances in IC design, silicon light sources have been integrated with the IC itself and this is referred to as an on-chip light source. A number of approaches exist where a silicon-compatible light source is integrated with a silicon substrate [1].
For complete integration, light sources should ideally be created using structures available in a standard CMOS/BiCMOS or SOI process used for typical ASIC integrated circuits. These include analogue, digital and mixed signal integrated circuits.
There are generally two methods by which to generate light in a standard CMOS integrated circuit where no hybridisation is necessary:                Radiative recombination of minority carriers through forward biased injection in a silicon pn-junction; and        Radiative carrier interactions under high electric fields, observable when inducing carrier transport through reverse biased pn-junctions in avalanche breakdown.        
The first technique emits light invisible to the human eye with a photon energy distribution around the band gap energy of silicon (approximately 1100 nm). The second technique emits a broad spectrum of light in the region of 400-900 nm, depending on the electric field profile. The current disclosure pertains to the second technique which emits visible light that is also capable of being detected by silicon itself. This is a unique feature of the disclosure described below.
Scientific advances in hot carrier electroluminescent light sources have enabled detectable light emission from light sources manufactured in silicon. Reducing the voltage and enabling circuit techniques that allow for direct interaction of these light sources with digital circuitry allow for the direct integration of these light sources as circuit components in analogue and digital systems on chip (SoC).
However, almost all of the on-chip light sources of which the Applicant is aware display an image or data (generally, an optical output) based on data which originates externally from the IC itself. In other words, the IC is configured to receive an input indicative of data to be displayed and then translate that input into an optical output via the on-chip light source.
The Applicant desires an IC having an on-chip light source which is operable to display information about the state of the chip itself. An IC, or integrated circuit, is seen as any semiconductor substrate which includes various electrical functions and circuitry, as well as additional functions such as electro-optical capabilities, integrated into a single body of that semiconductor. Examples include, but are not limited to, microprocessors, DRAM chips, dedicated EEPROMs and ASICs. Contactless probing has been explored using on-chip integrated light sources by a few authors in technical literature [2], but all of these rely on using on-chip silicon light sources in a single point in order to transmit information in a temporal one-dimensional manner, that is, by representing data in a linear time evolving sequence of optical pulses.
The Applicant accordingly desires a display arrangement which can display useful information about the state of the IC.